The present invention relates to devices for testing ring circuit continuity and more particularly to devices for testing continuity in ring circuits used for electricity distribution in buildings.
Final distribution circuits for electricity within buildings in some countries may be implemented using ring circuits. FIG. 1 shows a ring circuit 100 which comprises a multi-core cable, or multiple single core cables, from a consumer unit (or breaker panel) having an overcurrent protection device 130 for the ring circuit 100 to a plurality of accessories 102-124, such as electrical outlet sockets, the multi-core cable returning to the overcurrent protection device 130 and forming a ring connecting each of the accessories 102-124 and the overcurrent protection device 130. The multi-core cable typically comprises a Live, Neutral and Earth connection, also called Hot, Neutral and Ground in some countries. Countries in which such ring circuits can be found include the United Kingdom, Republic of Ireland, United Arab Emirates, Singapore and Indonesia.
Ring circuits 100 are used in order to save cost by reducing the amount of conductor, typically copper, used in the electrical installation and also to reduce the number of overcurrent protective devices 130 that are required. The amount of conductor is reduced because, although there is a need for the conductor to return to the consumer unit, each accessories 102-124 has two paths for the current to the appliance to flow. If the load of the accessories 102-124 is evenly distributed around the ring circuit, the current in each conductor is half of the total load current, allowing the use of cable with half the current-carrying capacity of the overcurrent protection device 130. In practice, the load does not always split between the conductors evenly, so thicker cable is used, typically rated at perhaps two thirds of the current-carrying capacity of the overcurrent protection device 130. The overcurrent protection device 130 may be rated at, for example 32A, whilst the cable may be rated at, for example, 21 A. Typically, the maximum load for each of the accessories is 13 A.
The number of overcurrent protective devices 130 protecting individual circuits may also be reduced because a single ring circuit 100 may supply a larger area than a single radial circuit where the cable starts at the consumer unit and ends at the accessory furthest from the consumer unit. Additionally, the flexibility of provision of power for multiple small consumers of power is improved because larger areas and more appliances can be served by a single ring circuit 100. The use of a ring circuit 100 can also help to reduce the voltage drop at appliances because of the additional path for current to flow.
Although ring circuits 100 are typically used for circuits for the connection of accessories 102-124 such as power sockets, they can also be used for external lighting which may cover an extremely large area including walkways and landscaped areas. The use of a ring circuit 100 in these applications may assist in reducing the voltage drop to required levels at a lower cost. Ring circuits 100 may also be useful for lighting in industrial buildings, warehouses, supermarkets and retail park outlets for the same reason.
FIG. 2 shows a problem which may occur with a ring circuit 200. If either the live or neutral cable is discontinuous at any one point 202, all of the accessories 102-124 will still function, but what was a complete ring circuit 200 is now two separate radial circuits connected to the same overcurrent protective device 130. The first radial circuit supplies most of the accessories 102-122 and the second radial circuit supplies only a single accessory 124. If the discontinuity is at the center of the ring circuit 200 and the load is distributed evenly around the ring circuit 200 there is unlikely to be a problem. However, if the discontinuity 202 is towards one end of the ring circuit 200, one cable will be taking the majority of the load current and risks being overloaded. Since the conductor in the cable is typically only rated at two thirds of the rating of the overcurrent protective device 130, there is a danger of cable overheating and even a fire occurring before the overcurrent protective device 130 cuts off the circuit. Even if the cable does not overheat, operating the cable above its rated current may result in reduced cable life. On a radial circuit such a problem would be apparent to users since outlets downstream of the discontinuity would not function.
Continuity of the ring circuit 100 may be determined by disconnecting the ring circuit from the consumer unit and measuring the resistance between each end of each of the wires in the circuit. If the ring circuit 100 is continuous, this will be a low value. If the ring circuit 100 is not continuous, this will be a high value. Another way of determining continuity of the ring circuit 100 is to remove each of the accessories 102-124 and check that the two portions of the ring circuit 100 are connected. Either of these tests carries the risk that the act of replacing the wires in the consumer unit after measuring the resistance or replacing the accessory 102-124 may itself result in a connection being broken.